Plant Gene and Trait 2024, Vol.15, No.6, 314-322 http://genbreedpublisher.com/index.php/pgt 316 3.3 Expression pattern of GRA78 The researchers used quantitative PCR and in situ hybridization to analyze the expression of GRA78. The results showed that GRA78 was mainly expressed in leaves, and the expression level was highest in the early growth stage of seedlings. The expression of GRA78 is also affected by environmental factors, such as light intensity and sulfur supply, which can affect cysteine metabolism. Interestingly, the expression of GRA78 will increase significantly when encountering oxidative stress, which indicates that it may also play a role in plant defense responses. These findings tell us that GRA78 not only affects the color of leaves, but also may help plants better cope with external stress (Gao et al., 2019). 4 Molecular Mechanism of Leaf Color Regulation 4.1 Biochemical pathways involved inGRA78 The GRA78 gene encodes a protein that is presumed to be S-thiocysteine synthetase. This enzyme is very important in the synthesis of cysteine. Cysteine is an essential amino acid that is essential for the normal functioning of cells. Cysteine is generated by the combination of sulfide and O-acetylserine, which is catalyzed by O-acetylserine (thiol) lyase (OASTL). In rice, the GRA78 gene is expressed in all tissues. The protein it encodes is sent to chloroplasts, indicating that it is related to the development and function of chloroplasts. In the GRA78 mutant, the expression levels of other OASTL homologous genes and photosynthesis-related genes were significantly reduced. This shows that GRA78 is very important for maintaining the normal functioning of chloroplasts and leaf color (Zhou et al., 2020). 4.2 Interaction of GRA78 with other genes and proteins The coordination between GRA78 and other genes and proteins also plays a major role in controlling leaf color. Serine O-acetyltransferase (SAT) and O-acetylserine sulfohydrolase (OASS) can combine to form a cysteine synthesis complex (CSC). This complex can sense sulfur levels in cells and regulate sulfur metabolism (Kumaran et al., 2009). CSC can enhance the activity of SAT and reduce the inhibition of cysteine on the synthesis process, so that cysteine can continue to be synthesized. In addition, the expression of GRA78 and its homologous genes is also affected by environmental conditions such as light, sulfur and nitrogen, which can further affect the color changes of leaves (Nakamura et al., 1999). 4.3 The role of plant hormones in leaf color regulation Plant hormones can also regulate leaf color by affecting genes related to chloroplast development. For example, in rice, there is a gene called GROWTH-REGULATING FACTOR7 (OsGRF7). It can change plant growth and leaf shape by regulating the metabolism of gibberellins (GA) and auxins (Figure 1) (Chen et al., 2020). OsGRF7 can directly bind to the promoters of genes related to GA synthesis and auxin signaling pathways, thereby affecting the hormone content and the plant’s response to external hormones (Chen et al., 2020). These regulatory processes indicate that there is a complex interaction between plant hormones and genes, and this relationship is very important in determining leaf color and overall plant growth. 5 Functional Study of GRA78 5.1 Gene knockout and overexpression experiments Regarding the function of the GRA78 gene in rice, the study mainly conducted gene knockout and overexpression experiments to clarify its role in chloroplast development and leaf color change. The GRA78 gene encodes a putative S-thiocysteine synthetase, which is very important for the production of cysteine, which is the raw material of many biological molecules. In a study, scientists isolated a green reversible albino mutant GRA78. The leaves of this mutant are white in the early seedling stage, but they will gradually turn green later. This phenomenon is affected by temperature but has nothing to do with light exposure time. Through map-based cloning analysis, it was found that the gene corresponding to GRA78 is LOC_Os01g59920, and its changes lead to this albino phenomenon. Subsequent complementation experiments also proved that the mutation of LOC_Os01g59920 did cause the phenotype of the mutant (Hao et al., 2022).
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